There are many applications in agriculture in which drawn implements are pulled behind tractors to move the implements over a desired path in order, for example, to pick up crop from a field, to mechanically cultivate the soil of a field or to discharge materials over a field. The tractor is customarily steered by a driver, and the implement attached to a coupling of the tractor by means of a draw bar follows the tractor. In particular in the case of relatively large tractors and implements, the dimensions of the tractor/implement combination, in particular the length of the draw bar of the implement, mean that a relatively large amount of space is required when turning in the headland. Because the headland has to be configured to be of an appropriate size; accordingly, a large area of the field for growing crops is lost. In addition, the soil is undesirably compacted on the headland which is traveled over relatively frequently. Attempts to reduce the size of the headland by means of greater steering angles of the tractor may result in damage if the drawbar or the implement collides with the tractor.
It has been proposed to provide a baler with a steerable drawbar or steerable wheels in order to automatically steer said baler in the lateral direction over a swath and to obtain bales which are as cylindrical as possible (e.g., U.S. Pat. No. 4,433,533). Furthermore, mounted implements having wheels driven by electric motor are also known (e.g., German Patent Application No. DE 196 23 738 A1). The wheels are driven according to the Ackermann condition, i.e. all of the wheels transmit identical forces.
German Patent Application No. DE 197 46 927 A1 proposes a soil-cultivating implement, in which the draw bar is mounted pivotably on the frame of the implement and the movement of the draw bar is transmitted in a mechanical, electric or hydraulic manner to a steerable main wheel. In this case, the main wheel is locked in the direction in which the draw bar moves, which leads to an improvement in the cornering characteristics.
European Patent Application No. EP 1 336 549 A1 describes a drawn agricultural distributing machine with a positively steered draw bar and wheels which are steered by assigned actuators and are activated by means of a controller in such a manner that the implement precisely follows the track of the tractor.
However, the turning circle of the implement is not positively influenced, i.e. with the effect of reducing the size thereof, by either solution, in particular if regions of the implement may collide with the tractor.
Finally, couplings on tractors have been proposed, which couplings are adjusted actively in the lateral direction by an actuator in order to move the implement over a desired path and, for example, to compensate for the downward slip on lateral slopes (e.g., U.S. Pat. No. 7,162,348).
However, during turning, the turning circle of the combination of tractor and implement is increased further by the activation of the coupling in relation to an implement without active steering.